Sorption mechanisms of zinc to calcium silicate hydrate: X-ray absorption fine structure (XAFS) investigation

In this study, X-ray absorption fine structure (XAFS) spectroscopy has been used to further elucidate the binding mechanisms of Zn(II) to calcium sili cate hydrate (C-S-H), the quantitatively most important cement mineral. Suc h knowledge is essential for the assessment of the longterm behavior of cem ent-stabilized waste materials. XAFS spectra of the Zn(II) equilibrated wit h C-S-H(I) for up to 28 days are best modeled by tetrahedral coordination o f Zn(II) by four 0 atoms in the first atomic shell. Beyond the first coordi nation shell, data analysis of more highly concentrated samples suggests th e presence of two distinct Zn distances and possibly the presence of an Si shell. On the basis of the comparison with a set of reference compounds, th is coordination environment can be reasonably related to the structure of h emimorphite, a naturally occurring zinc silicate, and/or the presence of ga mma -Zn(OH)(2). At the lowest Zn uptake, the above fitting approach failed and data could be described best with a Zn-Si and a Zn-Ca shell. Previous w ork has been able to show that Zn(II) diffuses into the C-S-H(II) particles and does not form discrete precipitates, so the findings appear to confirm the incorporation of Zn(II) in the interlayer of C-S-H(I).